Molar MassMolar mass of sulphur dioxide (SO2)
The molar mass of sulphur dioxide, SO2, is 64.06 g/mol. One mole of it, which is 6.022 × 1023 molecules, has a mass of 64.06 grams.
64.06 g/mol
32.06 + 2 × 15.999 = 64.058 · 3 atoms in the formula · also called sulfur dioxide
About sulphur dioxide
Sulphur dioxide is a choking gas formed when sulphur or fuels containing sulphur burn. It is a cause of acid rain and is used as a preservative and bleach. Its molecule is almost exactly half sulphur and half oxygen by mass.
Other names: sulfur dioxide.
The working
The formula SO2 has 1 sulphur atom and 2 oxygen atoms, 3 atoms in all. Multiply each count by the atomic weight of the element and add:
| Element | Atoms | Atomic weight | Total | Share by mass |
|---|---|---|---|---|
| Sulphur (S) | 1 | 32.06 | 32.060 | 50.05% |
| Oxygen (O) | 2 | 15.999 | 31.998 | 49.95% |
32.06 + 2 × 15.999 = 64.058 g/mol
With the rounded atomic masses used in many school books (S = 32, O = 16) the answer is 64 g/mol. The difference from 64.06 is too small to matter in most school sums.
Percentage composition
By mass, SO2 is 50.05% sulphur and 49.95% oxygen. The largest share comes from sulphur. In a 100 g sample there are 50.0 g of sulphur and 50.0 g of oxygen.
Notice that oxygen has more atoms in the formula (2) than sulphur (1), yet gives a smaller share of the mass. Percentage by mass depends on how heavy each atom is, not only on how many there are.
Worked examples
Grams to moles. How many moles are in 250 g of SO2? Divide the mass by the molar mass: 250 ÷ 64.06 = 3.903 mol.
Moles to grams. What is the mass of 0.2 mol of SO2? Multiply: 0.2 × 64.06 = 12.81 g.
Counting particles. 250 g contains 3.903 mol × 6.022 × 1023 = 2.350 × 1024 molecules, and 7.051 × 1024 atoms in all.
As a gas. At 0 °C and 1 atmosphere one mole of an ideal gas occupies 22.4 litres, so the density of SO2 is about 64.06 ÷ 22.4 = 2.86 g per litre. Air averages about 29 g/mol, so this gas is heavier than air.
Quick conversion tables
| Amount | Mass |
|---|---|
| 0.1 mol | 6.406 g |
| 0.5 mol | 32.03 g |
| 1 mol | 64.06 g |
| 2 mol | 128.1 g |
| 5 mol | 320.3 g |
| Mass | Moles |
|---|---|
| 1 g | 0.01561 mol |
| 10 g | 0.1561 mol |
| 100 g | 1.561 mol |
| 1 kg | 15.61 mol |
For other amounts use the grams and moles converter.
Questions people ask
What is the molar mass of SO2?
The molar mass of sulphur dioxide (SO2) is 64.06 g/mol, or 64.058 g/mol before rounding, using IUPAC abridged atomic weights.
How many moles are in 100 g of SO2?
100 ÷ 64.06 = 1.561 mol.
What is the mass of one molecule of SO2?
Divide the molar mass by the Avogadro constant: 64.06 ÷ 6.022 × 1023 = 1.064 × 10-22 g.
What is the percentage of sulphur in SO2?
Sulphur makes up 50.05% of the mass of SO2: 1 × 32.06 ÷ 64.058 × 100.
Which values? This site uses the abridged standard atomic weights published by IUPAC (CIAAW), read on 12 November 2026. Your textbook or question paper may round them differently, for example H = 1 and O = 16, so its answer can differ slightly. In an exam, use the values the paper gives.